Influence of sky conditions on carbon dioxide uptake by forests

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Abstract

Sky conditions play an important role in the Earth’s climate system, altering the
solar radiation reaching the Earth’s surface and determining the fraction of
incoming direct and diffuse radiation. Sky conditions dictate the radiation
distribution inside plant canopies and also the carbon dioxide uptake by forests
during the growing season. On the long term these diffuse conditions may have a
positive influence on forest growth in Northern Britain during the last 50 years.
We compared the quantity (amount) and quality (spectral distribution) of direct and
diffuse radiation above, inside and below a forest stand under sunny, cloudy and
overcast conditions in a thinned Sitka spruce [Picea sitchensis (Bong.) Carr.] forest (28
years, with an leaf area index (LAI) of around 5 m2m-2). Similar radiation properties
(sky conditions) were used for analysis of light response and canopy conductance
measurements in the same and also in a different spruce forest of the same species
(33 years, LAI of around 7 m2 m-2) over the growing season 2008 in order to compare
canopy activity under these conditions. In order to integrate short-term and longterm
studies, we were looking at how far these conditions are influencing forest
growth over several decades. To do so, we used freshly cut tree discs of Sitka spruce
from a felled forest (planting year 1953) in southern Scotland and solar direct and
diffuse radiation along with other meteorological data from the nearest
meteorological station.
Our analysis show that the amount and quality of solar radiation is distributed
differently inside forest stands under various sky conditions, leading to an
enhanced carbon dioxide uptake and canopy stomatal activity under diffuse cloudy
and overcast conditions. Furthermore we demonstrated which factors have
influenced diffuse radiation distribution over the past 50 years and how these are
correlated with forest growth in southern Scotland.